The present invention relates to inhibition of polymerization of monomeric aromatic compounds during the distillation thereof. More particularly, it relates to recovery and re-use of dinitrophenol compounds employed to inhibit the polymerization of styrene during distillation.
Vinyl aromatic compounds, such as styrene, its homologs and analogs, tend to polymerize upon standing at ordinary temperatures and particularly when subjected to elevated temperatures. Concentration and purification of these monomers is ordinarily effected by distillation or fractionation and in these operations considerable difficulty can be encountered due to polymer formation. The polymer can be deposited upon the surfaces of the equipment and accumulate to the extent that the distillation must be interrupted at frequent intervals to clean out the equipment. In addition to the fact that this would result in substantial losses of the monomer, the delays in production caused by the necessity of removing the polymer formed could also be an important cost factor. It is, accordingly, essential to prevent premature polymerization and in common practice this is done by employing a polymerization inhibitor during processing of monomers such as styrene by distillation and fractionation.
A large number of chemical compounds are known to be effective for use as polymerization inhibitors for vinyl aromatic compounds and, in particular, for styrene. In their book "Styrene, Its Polymers, Copolymers and Derivatives," Part I, p. 262, Boundy and Boyer disclose that ortho-nitrophenols are very good retarders of polymerization at elevated temperatures. S. G. Foord, J. Chem. Soc., 48, 56 (1940) in a study of a large number of polymerization inhibitors tested with styrene reports that one nitro group in an aromatic compound gives strong retardation of polymerization, two nitro groups have a stronger inhibiting effect and three nitro groups an even stronger effect. Thus, the use of dinitrophenols to prevent polymerization of styrene during distillation is well known in the art.
In the course of processing or purifying styrene using such inhibitors, certain amounts of the inhibitor normally accumulate in the still residues of the product styrene column which are commonly referred to as styrene tar or tars and are disposed of as waste, usually by burning. The phenolic compounds are, however, sufficiently expensive to justify their recovery for re-use. It is, accordingly, an object of the present invention to provide a process for recovery of dinitrophenol inhibitors used in styrene distillation in a form suitable for re-use in the styrene purification system. In fact, the recovery method of the present invention provides a solution of dinitrophenol inhibitors which is more effective than the commercial dinitrophenols ordinarily employed in retarding polymerization during the distillation of styrene. Other objects and advantages of the invention will become apparent from the following description thereof.